This invention relates to CAN bus termination circuits and to CAN bus auto-termination methods.
The Controller Area Network (CAN) protocol is a serial communication protocol for communicating between various electronic devices or nodes. CAN protocol has been used, in the past, in connection with vehicles such as automobiles. In accordance with CAN protocol, multiple different electronic devices or nodes can be coupled to a single serial bus, such that messages and data may be sent from one electronic device to another. The CAN protocol is a message-based protocol wherein CAN frames are placed on a common CAN bus. The CAN bus may be a single wire or may be a differentially driven pair of wires. Each electronic device or node on the common CAN bus receives each frame present on the bus and filters out those frames that are not required in performing that node""s tasks. For example, if a device associated with an automobile dashboard sends onto the CAN bus a frame requesting that the automobile headlights be turned on, then the device on the CAN bus responsible for the brake lights can determine that the frame is intended for another device and therefore will not act upon the frame. The device controlling the headlights, however, receives and acts upon the frame by turning the headlights on. Identifier bits are therefore provided in CAN frames to allow messages and data to be directed to certain nodes on the CAN bus, and not to other nodes on the CAN bus.
By virtue of the fact that signals are propagated on a common bus, noise and signal reflections are always a concern. Specifically, noise can corrupt signals carried on the CAN bus thereby leading to, in some instances, erroneous device behavior. In the same way, reflected signals can also compromise the integrity the system. To address signal reflection concerns, resistors, e.g. 120 ohm resistors, are typically incorporated into the CAN bus circuit at the ends of the CAN bus. Resistors must also typically be provided to satisfy driver requirements of a resistive load, e.g. in the ISO11898, the drivers require 60 ohms of resistive load. The requires finding the last nodes, e.g. at each end, on a CAN bus, and adding a termination resistor to the ends. Because CAN bus nodes are connected in parallel, it is not always an easy task to find the last node. Moreover, where an undetermined number of nodes are present, determining what devices are at the ends of the CAN bus is not an easy task.
This invention arose out of concerns associated with providing CAN bus systems and methods that are more flexible and adaptable than those that currently exist.
CAN bus termination circuits, and CAN bus auto-termination methods are described.
In one embodiment, a CAN bus termination circuit includes a first circuit in electrical communication with a CAN bus. The first circuit is configured to monitor an electrical operating parameter associated with operation of the CAN bus. A second circuit is in electrical communication with the first circuit, and is selectively enabled by the first circuit responsive to the first circuit""s sensing a change in the electrical operating parameter that it is configured to monitor. When enabled, the second circuit terminates the CAN bus.
In another embodiment, a CAN bus system includes a CAN bus and a plurality of nodes distributed along and operably connected with the CAN bus. A power-on signal generation circuit has an output line that is coupled with the nodes and enables the nodes. Each node includes a termination circuit that is connected to the output line. The termination circuit monitors for a predetermined electrical condition, and responsive to sensing the predetermined electrical condition, terminates the CAN bus.
In another embodiment, a CAN bus auto-termination method includes the steps of monitoring an electrical operating parameter associated with operation of at least one CAN bus node of a CAN bus, and terminating the CAN bus responsive to an occurrence of a determinable characteristic of the electrical operating parameter.